1. Field of the Invention
The invention concerns a method of treating a gas based on fluorine, obtained by electrolysis from baths containing hydrofluoric acid, and which may contain gaseous uranium compounds such as uranium hexafluoride. The method enables the constituents of the gas to be recovered separately, for example hydrofluoric acid which is carried over in electrolysis or uranium hexafluoride gas emanating from its own production process.
2. Description of Related Art
It is known that fluorine gas is generally obtained by electrolysis of anhydrous hydrofluoric acid dissolved in a bath of melted fluorides, that the fluorine thus produced carries over some 5 to 15% by volume of HF, due to the partial pressure of the HF in the bath at the electrolysis temperature, and that the HF may be a troublesome impurity in subsequent applications of the fluorine.
It is also known that the applications of fluorine include use in burning solid uranium compounds to obtain gaseous uranium hexafluoride. The solid compounds are chiefly uranium tetrafluoride (UF.sub.4) but also oxides or oxyfluorides where U is in the IV or VI state.
Such combustion is generally carried out with an excess of fluorine (6 to 8% relative to U), first in a primary reactor followed by a UF.sub.6 crystallizer (condenser), the non-converted or non-burned compounds (about 5% of the uranium involved) then being exhausted in a secondary reactor, again followed by a UF.sub.6 crystallizer. The pure uranium hexafluoride is condensed in the solid state in the crystallizers by refrigerating it to as low as -30.degree. C. HF is not condensed in the liquid state at this temperature in view of its low partial pressure.
A gaseous residue is recovered after combustion and crystallisation, including the excess fluorine, hydrofluoric acid from electrolysis which was not involved in combustion, non-condensed uranium hexafluoride and non-condensable gases, such as oxygen from the oxides introduced and/or nitrogen from various purges of the gas circuits.
The gaseous residue is normally treated--after being carefully filtered and passed into a non-return cold trap (at about -15.degree. C.)--by bringing it down (abattage) in a column using a basic solution, generally a potassium one, which leaves all the fluorine in solution in fluoride form and precipitates the uranium in the form of uranate contaminated with fluoride. The uranate cannot be recovered without undergoing a full, lengthy conversion treatment.
The fluoride solution in turn is generally treated with lime. This precipitates calcium fluoride free from uranium, which can be discarded after filtration, and regenerates caustic potash ready for recycling.
It will be appreciated that the treatment of the residue is long and difficult, that it condenses valuable products (UF.sub.6, F.sub.2, HF) in degraded or non-recoverable forms, that it consumes caustic potash, an expensive reagent, as a back-up material (appoint), and that there is always a danger of the discarded calcium fluoride being polluted with uranium. In addition, all the fluorine ends up in the form of discarded fluorides; this represents a large amount of effluent which is expensive to store, and a loss of fluorine which is also expensive.